Optical imaging systems generally include one or more externally mounted optical elements which shield the remainder of the imaging system from an external environment. For example, with infrared (IR) airborne imaging systems, an IR transparent optical element such as a window or dome is generally mounted on the airborne system to isolate the remainder of the IR imaging system from exposure to humid, corrosive, and/or abrasive environments. Prolonged exposure to these environments generally degrades the optical and physical characteristics of the material of the external optical element. In certain instances the most severe environmental exposure encountered by such external optical elements is high velocity water droplet impact that occurs when an airborne system is flown through a rain field. In addition, external optical elements are harmed by dust particles, such as sand, which may occur in desert environments.
In general, exposure to water droplet impact is referred to as rain erosion. During flight through a rain field, water droplets from a rain field impinge upon the surface of the external element producing subsurface fractures even at subsonic velocities. For very brittle materials, these subsurface fractures are initiated at pre-existing microflaws lying near or at the surface of the optical element. Rain erosion damage to such optical elements occurs prior to any significant removal of material. The mere propagation of these pre-existing microflaws is sufficient to damage the optical element. In particular, these microflaws are propagated through the optical element by the tensile component of a surface stress wave created at the time of impact with the water droplet. Once formed, the continued propagation of a subsurface fracture through the optical element will often produce large cracks in the optical element. In the region of the cracks, scattering and refraction of incident IR energy will occur that ends up producing increased internal reflections and IR energy losses. With a significant number of such cracks, the transmissivity of the optical element is severely reduced. Furthermore, as cracks propagate through the optical element, catastrophic failure of the element may occur. When the optical element shatters or breaks, the remaining optical elements of the IR imaging system are exposed to the external environment, resulting in potential catastrophic damage to the imaging system. Similar types of problems may also be caused by abrasion from sand particles. Even further, for airborne systems such as aircraft or missiles, damage to the window or dome may cause loss of control of the airborne system, which may be catastrophic.
Non-limiting examples of materials which offer the best mechanical durability and optical performance for infrared imaging systems, such as long wavelength infrared (LWIR) energy in the 8.0 micron to 12.0 micron infrared band, include zinc sulfide (ZnS), zinc selenide (ZnSe), germanium (Ge), gallium arsenide (GaAs), gallium phosphide (GaP), mercury cadmium telluride (HgCdTe), and cadmium telluride (CdTe). However, these materials are relatively brittle and have a relatively low resistance to damage, particularly damage sustained during high velocity impact from water droplets and dust particles, such as sand. For example, ZnS and ZnSe are relatively soft and lack durability when they are exposed to severe environmental conditions. To further complicate matters, coating materials that are hard may also be more absorbing, in particular at LWIR wavelengths. In addition, rain enhanced protective (REP) ZnS coatings deposited using radio frequency (RF) magnetron sputtering result in highly compressive stressed films that tend to delaminate from the base material during impact.
Optical energy incident upon a surface of an optical element results in reflection of the energy at the surface if the index of refraction of the material comprising the optical element is significantly different than the index of refraction of the medium from which the energy originates. Generally, for airborne systems, the originating medium is air having an index of refraction of about one. Accordingly, it is desired to provide optical elements and coatings using materials of appropriate refractive index to reduce losses attributed to reflection.